A clinical trial has shown that the combination cholesterol drug Vytorin is no more effective than a high dose of one of its components available generically, Vytorin's makers said Monday.

Vytorin, developed by Merck & Co. and Schering-Plough Corp., is a combination of Zetia and Merck's Zocor, which lost patent protection in 2006. Amid concerns about whether Vytorin posed a risk of liver damage, Wall Street has been anxiously awaiting details on results of the study begun in 2002.

In the quarter ended Sept. 30, sales of Zetia and Vytorin hit $1.3 billion, up 26 percent from the year-ago period.

Merck and Schering-Plough market Zetia and Vytorin jointly and split the profits. Shares of both companies fell Monday.

The study measured the amount of artery-clogging plaque in three areas.

It focused on a group of 720 patients with a rare condition predisposing them to high cholesterol. The patients were given either Vytorin or a high dose of generic Zocor, known as simvastatin.

In December, a congressional committee requested more information on the study. The results were delayed, the companies maintained, because of the complexity of the data.

Goldman Sachs analyst James Kelly reaffirmed a "buy" rating on Schering-Plough, calling the results a "non-event" that don't represent Vytorin's commercial prospects. He said the results reaffirmed the drug's safety.

However, Banc of America Securities analyst Chris Schott reaffirmed a "neutral" rating on Schering-Plough. He said the results raise several questions about Vytorin's effectiveness, which will likely go unanswered until the company presents the data at the annual American College of Cardiology meeting in March.

The patients in the two-year study had rare condition called heterozygous familial hypercholesterolemia, which the companies said affected about less than 1 percent of the population.

Earlier this month, Merck CEO Richard T. Clark said he expected Vytorin to keep a secure position in the cholesterol drug market and cited several other ongoing studies in addition to this one.

Merck and Schering-Plough are conducting three additional studies, involving more than 20,000 high-risk patients.